docs: Add migration guide to v3

Author: rkunev

README.md

@@ -60,6 +60,7 @@ The right color picker, but not the framework you're looking for?
 * [Usage](#usage)
 * [Options](#options)
 * [Change log](#change-log)
+* [Migration from v2](#migration-from-v2)
 * [Migration from v1](#migration-from-v1)
 * [Contributing](#contributing)
 * [Credits](#credits)
@@ -144,6 +145,27 @@ Please see [Releases][link-releases] for more information on what has changed re
 
 [Back To Top](#quick-links)
 
+## Migration from v2
+
+With v3 the inner circle is now transparent instead of solid white. If you previously relied on that and you prefer to keep the existing behavior you can do that by wrapping the `<color-picker>` with a `<div>` and add white background to it. Here's an example how to do that:
+
+```vue
+<template>
+    <div class="wrapper">
+        <color-picker :hue="hue" @input="onInput"></color-picker>
+    </div>
+</template>
+
+<style>
+    .wrapper {
+        padding: 32px;
+        background: #fff;
+    }
+</style>
+```
+
+[Back To Top](#quick-links)
+
 ## Migration from v1
 
 Straight forward - `v-model` becomes `v-bind` and you need to add the `@input` event (which was previously added by the `v-model` directive implicitly).